1. Field of the Invention
This invention relates to a clock-synchronized serial data communication system which is provided for communication between a camera possessing a microcomputer or the like and certain accessories (strobe, lens, etc.) that may be attached to the camera and, more particularly, to a system of this type capable of preventing any malfunction in data communication.
2. Description of the Prior Art
Systems for performing data communication between devices have been known, in the past. For example, U.S. Pat. Nos. 4,320,518 and 4,367,936 disclose data communication between two circuits, U.S. Pat. No. 4,588,274 discloses communication between a data transcribing apparatus and a camera, U.S. Pat. No. 4,512,644 discloses data communication from a camera to a strobe, and U.S. Ser. No. 688,355 discloses data communication between a camera and a lens system in which data is interchanged between the camera and the lens system in synchronism with clock pulses. These examples do not disclose any techniques for resetting data when a data communication cannot be performed normally. The last above-mentioned type of the methods of performing clock-synchronized serial data communication between different pieces of equipment such as a camera, a strobe and a lens system makes use of a data communication system such as that shown in FIG. 1.
FIG. 1 shows microcomputers I and II which are provided in a camera and an associated accessory, respectively. The microcomputers are interconnected through terminals a to d. The terminal a forms a clock pulse line. In synchronism with the supply of a clock pulse on this line, data is transmitted from the microcomputer I to the microcomputer II or from the microcomputer II to the microcomputer I via data lines b and c, respectively, thereby effecting intercommunication between these computers. A shift register SB which is provided in the microcomputer II into which items of data is serially input via the line b in synchronism with clock pulses supplied via the line a, as well as data in the register SB, is transmitted to the microcomputer I via the line c. A 3-bit counter completion of supply of 8 clock pulses effected via the line a. A reference symbol d denotes a ground line.
C N T   i s s u e s   a   c o m m u n i c a t i o n   c o m p l e t i o n   s i g n a l   w h e n   i t   d e t e c t s   t h e
In this arrangement, at the time of communication, clock pulses are sent from a serial clock terminal SC of the microcomputer I to a serial clock terminal SC of the microcomputer II via the line a.
Simultaneously, items of data from a serial output SO of the microcomputer I are transmitted to a serial input SI of the microcomputer II in synchronism with the clock pulses and are successively input into the shift register SB. The shift register SB is thus supplied with data from the microcomputer I, and successively transmits items of data which have been stored in it to the serial input terminal SI of the microcomputer I via the serial output terminal SO and the line c in synchronism with the above-described clock, thus effecting intercommunication between the microcomputers I and II.
Since the shift register SB is constructed with an 8-bit configuration, the above communication data is also provided in the 8-bit form. The counter
C N T   d e t e c t s   t h a t   d a t a .
That is, the counter CNT is a 3-bit counter which outputs a communication completion signal INTS by counting 8 clock pulses. This system, therefore, detects data communication.
As described above, in this type of system, data communication is performed on the basis of clock pulses supplied via the clock line a. Therefore, if noise occurs on the clock line a, there is a possibility of the shift register SB starting a shifting operation and, hence, a change in the set data in the shift register SB.